The importance of understanding the controls governing commitment and differentiation of hematopoietic stem cells (HSCs) to specific lineages is underscored by the fact that a block in differentiation is a hallmark of acute leukemias. Acute myeloid leukemia (AML) accounts for most acute leukemias in adults. The transcription factor PU.1 regulates nearly every known myeloid gene and is absolutely required for normal myeloid development. PU.1 is expressed in stem cells and up-regulated early during myeloid and lymphoid commitment. The importance of understanding how PU.1 is regulated is emphasized by studies indicating that altered expression of PU.1 can induce changes in hematopoietic lineage development and dysregulation leads to leukemia. Thus, the overall goal of this continuation proposal is to carry on our studies of how PU.1 is regulated. In the next funding period, we plan to continue previous efforts to study how transcription factors regulate PU.1 by binding to the PU.1 upstream regulatory element (URE). In addition, we will extend these studies to the role of proper regulation of PU.1 on HSC function. These studies will further our understanding of commitment of normal hematopoietic precursors to the myeloid lineage. As such, they are relevant to understanding the block in normal myeloid maturation from blasts to mature myeloid cells in AML.